What do I need to add to improve this deck?

I think since you have water deck, there is three more cards that might be good for you. One card is kabasaurus, it is normal monster that have ATK of 1700 attribute of water and since you have The legendary ocean, Kabasaurus raise attack as well. Second card is Yomi ship, effect monster card with DEF of 1400, but when any monster that destroys this card, that monster automatically be destroyed also, this card is good for defending your life. Last card is also normal monster card Gogiga gagagigo, you have to sacrifice 2 monster and it has ATK of 2750or so. All of those monster cards will become strong when The legendary ocean card is activated.